The present invention relates to a method for a dryer section of a paper or board machine having a single-wire draw.
The present invention relates to a device in a dryer section of a paper or board machine having a single wire draw.
As is known from the prior art, in multi-cylinder dryers in paper machines, twinwire draw and/or single-wire draw is/are employed. In twin-wire draw, the groups of drying cylinders include two wires, which press the web, one from above and the other one from below, against heated cylinder faces. Between the rows of drying cylinders, which are usually horizontal rows, the web has free and unsupported draws, which are susceptible of fluttering, which may cause web breaks, in particular as the web is still relatively moist and, therefore, of low strength. This is why, in recent years, ever increasing use has been made of said single-wire draw, in which each group of drying cylinders has one drying wire only, on whose support the web runs through the whole group so that the drying wire presses the web on the drying cylinders against the heated cylinder faces, and on the reversing cylinders or rolls placed between the drying cylinders the web remains at the side of the outside curve. Thus, in single-wire draw, the drying cylinders are placed outside the wire loop, and the reversing cylinders or rolls inside the wire loop.
It is known from experience that, if paper is dried one-sidedly, the result is a tendency of curling of the sheet. When paper is dried by means of normal groups with single-wire draw from the side of its lower face, and if such asymmetric drying is extended over the entire length of the dryer section, the drying takes place so that first the side of the bottom face of the paper web is dried, and when the drying makes progress, the drying effect is also spread to the side of the top face of the paper web. Thus, the dried paper is, as a rule, curled so that it becomes concave when viewed from above. From the point of view of runnability of the paper machine, however, a dryer section with full support over its entire length and based on normal groups with single-wire draw, without inverted groups, would be a particularly justified solution.
With respect to the prior art related to the present invention, reference is also made to the U.S. Pat. No. 5,600,898, in which an arrangement related to the control of curling in the dryer section of a paper machine is described. It is a drawback in said arrangement that it does not permit the use of a runnability component operating with the principle of blowing, which is a definite requirement when running takes place with open wires at high speeds.
One of the objects of the present invention is to provide a solution which permits a dryer section based on normal groups with single-wire draw.
In the solution of method and solution of equipment in accordance with the present invention, in its commonest embodiment, the web support function and the web impingement drying function have been accomplished by means of the same solution of equipment, which comprises one continuous hood, i.e. box construction, for the device. In accordance with the invention, said equipment extends into the pocket space between the drying cylinders and the suction roll that operates as a reversing roll so that air is removed out of said pocket space and/or an ejection blowing is produced in said pocket space along the wire, in which case the web is affixed to the wire face by means of a vacuum. Since, favourably, wires of high permeability are employed, application of the vacuum to the web in this way is possible. In accordance with the invention, by means of the same solution of equipment extending into the pocket space, impingement drying is also carried out. Preferably air, favourably heated air, or steam is employed. Within the scope of the present invention, an embodiment is possible in which, through the impingement drying unit, a part of the impingement drying air is passed through the interior of the box construction to the end of the box construction and/or further as an ejection jet and/or as a closing jet and/or as an exhaust blow jet at the inlet and outlet side of the suction roll in the vicinity of the wire/web. Thus, in a preferred embodiment of the invention, the impingement drying hood extends onto the drying cylinder and further into the pocket space.
In the embodiments defined in the sub-claims of the invention, it is suggested that the device be employed in certain areas of the dryer section, for example in the areas of the dryer section in which the dry solids content of the web is higher than 60%. In such a case, impingement drying is employed in particular for prevention and control of the curl of the web.
The invention is characterized in what is stated in the patent claims.
The present invention is applied in a dryer section in which a normal single-wire draw is applied at least partially. The dryer section may also be such that it is provided with impingement drying units.
Out of the impingement drying units, hot air/steam is blown through the wire onto the paper. By means of such an impingement drying unit, a considerable increase in the evaporating capacity is achieved. In such a case, the evaporation takes place to an increasing extent from the wire side on the cylinder. Owing to the increased evaporating capacity, the dryer section can be made of shorter length, and in this way economies can be obtained in the cost of construction of the hall. Owing to the increased evaporating capacity, the concept of the present invention can also be applied to modernizations, in which the available space is often quite limited.
From the patent application JP 222 691/1993, a dryer section is known in which there is an impingement drying hood above all of the upper cylinders. Thus, it has been known from the prior art to enhance the evaporation taking place on a cylinder by ventilating the rear side of the wire or by blowing hot air partly through the drying wire.
By means of studies carried out on test devices, it has been established that the evaporating capacity that can be achieved depends highly extensively on the permeability of the wire. In order that an increase in the evaporating capacity could have economic significance, the permeability of the wire must be preferably in the range of 2000 . . . 20,000 m3/h/m2 (cubic metres per hour per square metre), preferably 4000 . . . 10,000 m3/h/m2. The permeability, i.e. permeability to air, of a wire H is the flow rate as cubic metres of air per hour that passes through an area of a size of one square metre of a wire when the pressure difference across the wire is 100 Pa.
Formation of pressure in a closing nip is prevented so efficiently that no bag formation in the web takes place. One possible alternative solution is a runnability component, which prevents passing of air into a closing nip by means of suction and permits the use of an open wire, in which case evaporation from the top side of the web can be enhanced, for example, by means of solutions of the type of impingement drying hoods. It is possible to employ a suction box that fills the whole pocket at the suction roll, which suction box is provided with preventive blowings at the edges in order to prevent leakage of air into the pocket.
In the preferred case, the necessary portion of the dryer section or the whole dryer section consists of assemblies with the construction described above. It is a great advantage of this solution that there is no need for an inverted group, which is difficult in view of cleaning, and it is, nevertheless, possible to dry the web efficiently and even to regulate the drying capacity at the top side and bottom side of the web by means of the speed and temperature of the impingement drying air.
In the present invention, impingement drying hoods are preferably placed in connection with those cylinders only at which they provide a considerable effect either in the control of curl or in increasing the drying capacity.
When the impingement drying is carried out through the wire, the paper is protected between the wire and the cylinder, and the paper cannot form folds which might collide against the impingement drying device and damage it.
Increased evaporation on the cylinder normally causes a lowering of the average temperature of the web and thereby somewhat reduces the evaporation taking place in the area of a free draw, but, on the other hand, the delivery of heat by the cylinder is increased, which increases the overall evaporation.
In the dryer section in a paper machine, in an area of single-wire draw, it is known from the prior art to employ various blow boxes or runnability components in order to improve the runnability of the dryer section. One such runnability component is described in the applicant""s U.S. Pat. No. 4,905,380 (FI Patent 80,491), in whose arrangement, in a multi-cylinder dryer in a paper machine, for the purpose of supporting the web, combined blow-suction boxes are employed, which have been fitted in the gaps between drying cylinders and which are provided with a plane wall at the inlet side of the drying wire and the web, a nozzle opening or openings being opened at the edge of the wall, by means of which opening(s) an ejecting flow is blown in the direction opposite to the direction of movement of the adjacent drying wire, by means of which ejecting flow a field of vacuum is induced in the gap space between said wall and the straight run of the drying wire and the web and in the following wedge space. The blow-suction boxes that are used include a suction and/or closing compartment, by whose means the free sectors at the top of the reversing cylinders between the adjacent wedge spaces have been covered. In said patent, a so-called box of a whole pocket is described, which fills substantially the entire pocket space while taking into account the necessary safety clearances.
It is a further object of the present invention to provide a solution in which, in the dryer section of a paper machine, the runnability is improved and, at the same time, the curl is controlled and the drying is enhanced, in particular in the dryer groups towards the final end of the dryer section.
It is an object of the present invention in particular to provide an arrangement which is suitable for use in connection with wires more open than usual at high running speeds of paper machines.
In a preferred embodiment of the method in accordance with the present invention, when the dry solids content of the web is higher than 60%, the web is dried through the wire by means of blowings produced by means of a blow box, by means of which blowings, on the straight runs of the paper web and the wire between the reversing cylinders or rolls and the drying cylinders, at the outlet side of the web and the wire, at the same time, the support contact between the paper web and the wire is enhanced in order to improve the runnability, and in the method a wire more open than usual is employed, whose permeability, i.e. penetrability to air, is 2000 . . . 20,000 m3/h/m2 and preferably 4000 . . . 10,000 m3/h/m2, in which case the drying of the web at the outlet side takes place both on the heated cylinder face of the drying cylinder and by means of drying blowings out of the blow box in view of controlling the tendency of curling of the web.
In a preferred solution of equipment of the present invention, blowings that dry the paper web have been fitted to be produced by means of a blow box at the outlet side, which blowings have been fitted to be blown towards the web through the wire, the permeability of said wire being 2000 . . . 20,000 m3/h/m2, and said blowings are applied to the web when its dry solids content has exceeded 60%.
According to an embodiment of the invention, in connection with dryer sections of paper machines that apply single-wire draw, a blow box or an equivalent runnability component is employed, by whose means, at the same time, improved runnability and control of curl and enhanced drying are achieved. The invention is applied in particular in the dryer groups towards the final end of the dryer section in a paper machine. The invention is applied in dryer groups in which the dry solids content of the paper web exceeds a desired limit value, for example, is higher than 60%, preferably 65%. The device in accordance with the invention comprises runnability nozzles and runnability/impingement drying nozzles, and in connection with the device in accordance with the invention, a drying wire is employed that is more open than normal, whose permeability is 2000 . . . 20,000 m3/h/m2, preferably 4000 . . . 10,000 m3/h/m2, in particular for paper machines at which high speeds are employed, for example 1000 . . . 2400 metres per min., preferably 1200 . . . 2000 metres per mm.
In the dryer groups in the initial part of the dryer section, most appropriately so-called blow boxes of a whole pocket space are used, which boxes are known, for example, from the applicant""s said U.S. Pat. No. 4,905,380, and from the desired dry solids content onwards, blow boxes in accordance with the present invention are used. In accordance with a preferred embodiment of the invention, the runnability/drying blowings at the opposite side are continued over the drying cylinder as impingement-drying/through-drying blowings extending onto said cylinder, by means of which blowings the control of curl is enhanced further.
In accordance with the present invention, a system is provided for two-sided drying, in which, in the drying area proper, in which the necessity of curl control is also emphasized, thus, more open wires are used, which permit blowing through the wire, and at the same time a blow box in accordance with the present invention is used for the control of runnability and curl.
According to a preferred embodiment of the invention, in connection with the method in accordance with the invention and in connection with embodiments of equipment in accordance with the invention, as the drying fabric, a wire is employed whose face has been treated in order to improve the holding of the web in contact with the wire. Such a what is called sticky wire further ensures the keeping of the web on the face of the open drying wire. One such sticky wire is the wire marketed by Albany International with the product name Aerogrip(trademark), and in respect of said wire reference is also made to the published EP Patent Application No. 0,761,872. A sticky wire can also be accomplished, for example, in compliance with the principles suggested in the U.S. Pat. No. 5,397,438 (equivalent to FI Patent 84,088).
By means of a blow box in accordance with the present invention, an efficient formation of a vacuum is produced at the inlet side by means of an ejection effect, and the nozzle that forms a vacuum at the opposite side, i.e. at the outlet side, also operates as a nozzle that enhances the drying of the web, which nozzle dries the web from the side opposite to the face dried by the cylinder face, whereby the curl of the web can be controlled. This blow face of the opposite side can also be brought as an extension of the box by means of a separate system of ducts, or a drying box or a chamber completely separate from the box at the opposite side can be formed.
Thus, at the inlet side, the device in accordance with the invention comprises a nozzle which blows in the direction opposite to the running direction of the web and which has been formed so that it blows into the opening passage in order to enhance the vacuum effect. The nozzles of the longitudinal direction can be provided separately with air ducts of their own. The blowings at the opposite side, or combinations of same, are formed so that they improve the runnability and enhance the drying of the web W, in which case, in the blowing, dry air is employed, and the blowing is preferably directed at the wire face, and the drying effect can be applied to the paper web through the wire that is more open than normally. When the device is composed of two separate boxes or chambers, at the inlet side preferably circulation air is used. The length of the blow face at the opposite side is not limited, but it may cover the cylinder over an area of up to 180xc2x0.